Analysis of the interaction of planetary plasma and dust with the magnetic field

点击翻译按钮获取中文摘要

Newest plasma data recorded by the Cassini spacecraft suggest that dust and plasma in Saturn s inner magnetosphere interact in a more complex manner than previously thought (Wahlund et al., 2009). The data was obtained during crossings of Saturn s E ring, a tenuous dust ring that is formed by submicron and micron sized grains (dust) of water ice (see cover figure). These grains are ejected by the active ice-moon Enceladus. The dust streams emerging from Enceladus were directly measured by the Cosmic Dust Analyzer (CDA) onboard the ongoing NASA/ESA Cassini-Huygens mission to Saturn (Spahn et al., 2006b; Schmidt et al., 2008). In orbit, the grains revolve under the influence of the planet s gravity, solar radiation, and the electromagnetic field in the Saturn system. They also interact with the plasma in Saturn s inner magnetosphere. The commonly accepted picture in the last decades was that corotational plasma charges the E ring grains (Horányi et al., 1992), which feel the effects of the induced corotational field (Birmingham and Northrop, 1979) and Saturn s magnetic field, while there is no significant feedback on the plasma flow. However, the plasma densities inferred by the Cassini Langmuir probe (Wahlund et al., 2009) show a clear misfit between the densities of electrons and ions (the latter are mostly positive and mainly single charged), which is seen when the spacecraft (vertically) crosses the E ring (Fig. 1, upper panel) near its densest part. Simultaneously, the measured ion drift speeds (Fig. 1, lower panel) are intermediate (20km/s, relative to the spacecraft) between Kepler speed (8km/s) and corotation (25km/s). A simple explanation is that a significant amount of the electrons rests on dust grains in Saturn s E ring, so that their charge is not directly registered by the Langmuir probe (Wahlund et al., 2009). In turn, the grains slow down the plasma flow via coulomb drag, offering an explanation for ion drift speeds slower than corotation. If this scenario is correct then the related currents in this region will induce deviations from the Saturnian magnetic field bearing consequences for the dynamics of the E ring dust, the evolution of Saturnian plasma, and for the interpretation of magnetic field measurements in this part of Saturn s magnetosphere. Preliminary estimates show that a large amount of small E ring grains is necessary to explain a charge misfit of the observed order, down to sizes between tens of nanometers and one hundred nanometers. This estimate employs a steep grain size distribution (Schmidt et al., 2008), together with the appropriate size-dependent equilibrium charges for a given potential in the E ring region of about -3V (Kempf et al., 2006). This is interesting, since also measurements by the Cassini Plasma Spectrometer (CAPS) instrument hint at the abundant production of grains of this size directly in the plume of Enceladus (Jones et al., 2009). Thus, two independent measurements point to an existence of a population of small E ring grains. Otherwise, this range of particle sizes is hard to constrain by in situ measurements by Cassini CDA or by photometry of the E ring, using images obtained by the Imaging Subsystem (ISS) or the Visual and Infrared Mapping Spectrometer (VIMS). In the proposed project we plan to extend existing models to estimate the configuration of E ring dust down to grain sizes of 10 nm, and estimate the interaction of dust and plasma, in order to derive a model for the related azimuthal currents in the plasma disk and for the so induced perturbations of the Saturnian magnetic field.